Martin WarneSenior Technical Sales Engineer

OTDR > Back To Basics

Tuesday 5th June 2018

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The Issues

All of these issues make OTDR testing as much of a headache as it was 25 years ago

Fiber Everywhere Set UpTrainingHigh speed Analysis

FTTH, DC, 5G, etc.

10G FTTH 400G Core. Reduced Needs to be

correct.Right or Wrong?

Remove macrobendsNot accounted for in loss budget and can go undetected. Impact DWDM and future FTTH technologies.

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Verify the network length as per loss budget

Verify connector loss AND reflectance.Dirty or damaged connectors have a high impact on loss budget and are often overlooked.

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1 Verify splicing standard is acceptable and within limits.Resplice if necessary

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Why OTDR?

Reflectometry TheoryOTDR launches short duration light pulses.

The pulses encounter scattering (Rayleigh) and Reflective (Fresnel) events. A fraction of the pulse returns to the launching port. The returned signal is proportional to pulse power and varies as a function of the event.

Measuring the difference between the launching time and the time of arrival of the returned signal, one determines the distance between the launching point and the event.

Lord John Strutt Rayleigh

English Physicist

1842 -1912

Augustin Jean Fresnel

French Physicist

1788 -1827

Reflectometry TheoryRayleigh Backscattering

Comes from the “Natural” reflection of the fiber

The OTDR will use the Rayleigh back reflections to measure fiber attenuation (dB/km)

Back reflection level around -77dB

Higher wavelength will be less attenuated by the Rayleigh Backscatter

Consider splices have ‘natural reflection’ also. Source

Ray of light

Silica particles

Fresnel back reflections

Will come from abrupt changes in the IOR, ex: (glass/air)Fiber break, mechanical splice, bulkheads.

Will show as a “spike” on the OTDR trace

UPC reflection is typically -55dB and APC -65dB (as per ITU)

Fresnel reflections are higher than natural backscatter

Will create a « Dead Zone » after the reflection

Reflectometry Theory

Distance calculation

Time is correlated to distance :D = (c t) / 2n

D : Distance along the fiberc : Speed of light (in a vacuum)t : Time of the round trip of the lightn : IOR

Reflectometry Theory

Corning SMF-28e+ ULL (G.652D) Corning LEAF (G.655)

A Successful Test Strategy

We do see:

Fiber Networks now everywhere, not just core, now -> 4G/5G, FTTH, DC, etc. Training reduced or even stopped altogether

Unskilled technicians working on the network Timescales are tight

Resulting in: Incorrect test sequences Misinterpretation of results Networks passed that should have failed Repeat truck rolls at some stage required

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OTDR: 2 Steps

Set UpOur result will only be as good as our set up. An educated set up will greatly assist in giving a more accurate picture of the

link we want to measure.

Incorrect set up or a dependency on the unit to make a decision for us:

AUTOWill give us a false sense of security that

the OTDR is making the optimum selection of Distance and Pulse Width

Reading the ResultHere lies the greatest cause of

issues, an inability to understand the result in front of you.

Many issues or problems are missed due to misinterpretation

of the result displayed.

Assistance is given in trying to detect macrobends, high losses, high reflectance but it ultimately is the users responsibility to P/F

the link.

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Step 1: Setting up the OTDRWavelengths

Use at least 2 wavelengths to capture macrobends. For long distances use

only 1550 and 1625nm if available.

Pulse WidthProbably the most

important setting. Will determine how much of the link we can see and in what

detail.

Distance/RangeUse a minimum of Launch Cable + Fibre Under Test +

25% for the distance to use.

Averaging/TimeWill allow a cleaner trace to

more accurately see the events. The minimum should

be to see clearly the final event before the end of fiber.

SettingsIoR, BsC, splice,

reflectance and EoFthresholds as well as

P/F thresholds.

Distance*

Pulse Width*

Averaging Time

FUT + 25% ≈Min 15s or 30s

???? > Loss Dependent

Step 2: Reading The Result - Tips

Reminder: Our OTDR Result is ONLY as good as our set up

Inspect and Clean ALL optical connectors on OTDR & ODF Use a Launch/Dummy Fibres to characterise Connector at ODFs Spend 5 – 10 minutes experimenting with settings to optimise

If necessary use AUTO as a first pass and adjust Review your OTDR trace from the START and not the END

Tendency is to look at END, anything lost at START….. Give result a correct naming convention Save in raw or .SOR format

Repeat for Opposite Direction if required (B>A).

Our Result

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1 > Launch/Dummy Fibre: Loss and Reflectance at ODF:

Our Result

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2 > Splices in the link: Loss > Losers and Gainers (due to MFD mismatch)

Our Result

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3 > Missing Splices in the link: Under Threshold Set

Our Result

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4 > End of Fiber: Reflectance only, no Receive/Dummy fiber

Our Result – But We Missed?

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1 > Patch at start of 5m2 > Missing Splices in the link

Pulse Width too large to capture

Ugly Stuff - Bends

Ugly Stuff - Ghosts

Inability to clean connectors leads to high reflectance and ghosting

FTTH NetworksDue to close nature of high loss splitters, may be necessary to do 2 traces to see both and close events.

Multiple Pulse Solutions Use multiple PW depending upon topology Allows sight of near and far end events

Self setting unit conditions Advanced algorithms to consolidate into one file Results displayed as icons and interpretation of trace taken by software not

individual. Can store as Multiple PW file, .SOR and .PDF.

How to See More?

How to See More > P2P Example

Now we can see the near end events and a macrobend > Analysed by the unit, not the individual

How to See More > FTTH Example

OTDR Test Summary?

OTDR Requires KnowledgeSetting Up The Unit

Correct Distance, PW and TimeUse AUTO under conditions!Interpretation

Multiple Pulse Width SolutionsNo set-up – self settingMultiple pulse widths/wavelengthsEasy/intuitive icon results displayReduced training requirements/experience

Q&A

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